Sharing email domain between clients

I have one domain that I have set up mailboxes for, each one for a different client. In this case this is actually for members of my family. I set all of these up using the admin account, and each of them has a corresponding client account (including myself).

All of these mailboxes are name@mysurname.com. I'd like each of them to be able to configure their own mailboxes in their own client accounts, and add new ones using the domain mysurname.com, but without seeing each others mailboxes.

Is this possible? I tried setting up another client and transferred ownership of mysurname.com to that temp client, then added everyone else to that group, but then all family members could see all mailboxes, definitely not ideal!

Thanks Till. I've just discovered that the Roundcube plugin has an account settings, and I think that gives these users most of the settings they need. It's an unusual use case, was just curious if ISPConfig catered for that sort of situation!